So I had a bunch of UI code styled with CSS in a Composite and everything was just fine. Specifically I had a nice gradient color on the Composite, and the labels in the composite had a background of 'inherit' which means the gradient shined through.
I decided to put that Composite into a ScrolledComposite because it was getting bigger than the bounding part. BANG! The background of the labels suddenly no longer inherits. It flips to the system default, painting nasty bars across the smooth gradient that was there before. No other code changes.
You can see before.png and after.png below.
The CSS for the labels is pretty simple: